We’ll close this chapter with a few examples of glycoproteins that play crucial roles in human physiology. Let’s look first at the major human blood groups. The major A, B, AB, O and Rh blood groups result from the presence or absence of glycoprotein antigens embedded in red blood cell membranes and the presence or absence in the blood, of antibodies against the antigens. Typically, exposure to antigens (foreign substances like bacteria, viruses, toxins…) generates immunoglobulins, the antibody molecules of our immune system; immunoglobulins are glycoproteins. The situation with blood groups is something of a paradox. The blood group antibodies already in the blood of a healthy person are not a response to foreign antigen invasion
You probably know that these blood groups must be compatible for a successful blood transfusion. A mismatch between donor and recipient can be devastating. The interaction of the red cell antigens of one blood group with antibodies in another blood group will cause the red cells to clump, restricting blood flow and ultimately killing the transfusion recipient. The table below summarizes why transfusions with mismatched A, B, AB, O blood groups must be avoided.
|Group A||Group B||Group AB||Group O|
|Antibodies inthe blood||
|Acceptable donor-recipient matches||Group A or Group O donors||Group B or Group O donors||Universal Recipient (Group AB, A, B, O donors)||Only Group O donors|
|Why red cells clump in mismatched blood||Anti-A from Group B donor binds, aggregates recipient red cells; recipient Anti B binds, aggregated donor red cells||Anti-B from Group A donor binds, aggregates recipient red cells; recipient Anti A binds, aggregates donor red cells||Antibodies in Group O blood will bind any donor red cell antigens and cause the cells to clump|
Another red blood cell antigen is the Rh factor. People have either it (Rh+ ) or not (Rh- ). In contrast, when an Rhrecipient receives blood from an Rh+ donor, the recipient’s immune system makes defensive anti-Rh antibodies in the usual way. This too can cause blood cell clumping with bad consequences. A word to the wise: it’s a good idea to know your own blood group!
Check the Red Cross website (here) or Wikipedia for more detail about blood groups.
The last example here involves the cell surface major histocompatibility complex (MHC) glycoproteins that distinguish self from non-self in body tissues and organs. Major organ Transplantation (liver, kidneys, heart) from donors into patients with failing organs has become, if not routine, then at least increasingly common. Before a transplant, MHC tissue typing determines donor and recipient compatibility, reducing the chances of the rejection of the transplanted organ. Since available donors are few, and good matches even fewer, patients wait on prioritized lists for a matched organ. Even when MHC typing is a match for a patient, the immune systems of transplant recipients are suppressed with hormones to reduce further the chance of rejection. Unlike the limited number of blood groups, many MHC proteins are analyzed to determine a match. Thus, it is not practical (or routinely necessary) to ‘know’ your MHC type!
In the next chapter, we look at membrane functions intrinsic to cellular existence itself.